1. Field of the Invention
This invention relates to a fluidized bed polymerization apparatus and an olefin polymerization process by using this apparatus, more especially, to a fluidized bed polymerization apparatus, with which an olefin polymer can be produced continuously for a long period of time, stably and reasonably, and an olefin polymerization process by using this apparatus.
The term “polymerization” used in this specification means polymerization including homopolymerization and copolymerization; “polymer”, polymer including homopolymer and copolymer.
2. Description of the Background
Olefin polymers are represented by polyethylene, polypropylene, and linear low density polyethylene, which is a copolymer of ethylene and α-olefin, and the like. These olefin polymers are widely used for film-forming material and the like.
Such olefin polymers can be produced by using a Ziegler-Natta catalyst or a metallocene catalyst. In recent years, properties of transition-metal compound ingredients contained in this catalyst have been improved. As a result, an olefin-polymerization activity per unit weight of a transition metal has been greatly increased. Consequently, operation of removing catalysts after polymerization reaction can be excluded. On this account, cases adopting a gas phase polymerization, in which polymerization operation is easy, by using this highly active catalyst has been increased.
In the conventional gas phase polymerization, such a fluidized bed gas phase polymerization vessel (reactor) equipped with a gas-dispersing plate as shown in FIG. 12 has been much used. In the gas phase polymerization with use of this polymerization vessel, olefin or olefin-containing gas is introduced through an introducing pipe 72 into the bottom 70a of the polymerization vessel by using a compressor or a blower 73. Then, the above gas is uniformly dispersed through the gas-dispersing plate 79, risen up in the polymerization vessel and brought into contact with catalytic particles in a fluidized bed region 70b located on the upper side of the gas-dispersing plate 79. Consequently, polymerization reaction is progressed in a fluid state.
In this case, an olefin polymer is formed on each surface of the catalytic particles. Accordingly, solid particles each composed of the catalytic particle and olefin polymer are floated in the fluidized bed region 70b. And the polymer particles can be discharged outside out of a discharging line 75 to be recovered. On the other hand, olefin polymer particles polymerized in the fluidized bed region 70b tends to be scattered into the upper part of the fluidized region 70b. In order to prevent the particles from scattering, a reduction region 70c through a gas-circulating pipe 76 with large cross section is provided in the upper part of the fluidized bed polymerization vessel 70 for reducing a gas flow speed. Unreacted or unpolymerized gas is discharged from the top of the reduction region 70c, cooled by using a heat exchanger 77 with cooling water, brine or the like, and again, charged into the bottom 70a of the fluidized bed polymerization vessel 70. Thus, the unreacted or unpolymerized gas is reused by circulation.
By the way, when such fluidized bed polymerization reactor 70 as had aforementioned is operated for a long period of time in order to perform continuous gas phase polymerization, there is some case that the following results will be caused.
(1) In case that the dispersion of the solid particles in the fluidized bed region is not uniform, the solid particles will adhere to the inner wall of the fluidized bed polymerization vessel 70. When polymerization reaction is progressed in this state, heat of polymerization cannot be removed sufficiently from the adhered portion. Consequently, temperature will rise locally at this portion.
(2) Olefin polymer particles will weld together to grow up into mass or sheet polymers. These grown up particles will be dropped and settled in the bottom of the fluidized bed polymerization vessel 70. Otherwise, these particles stay in the middle part. Consequently, temperatures at these local spots will be lowered.
(3) Moreover, when polymerization reaction is progressed promptly within a short period of time, the inner temperature of the polymerization vessel 70 will rise quickly to make progress of polymerization at abnormal speed. Especially, when sudden reaction is locally occurred, hot spots will be generated. This will produce new sheet or mass polymers, thereby operation of the polymerization reactor 70 become unstable.
As for a monitoring means of these occurrence, as shown in FIG. 12, in the previous time, two temperature measuring devices 78a and 78b have been put into the fluidized bed region 70b of the fluidized bed polymerization vessel 70. Further, one temperature measuring device 78c has been put into the reduction region 70c. By these temperature measuring devices the inner temperatures of the fluidized bed polymerization vessel 70 have been checked. In other words, when something unusual in the temperatures, measured by these temperature measuring devices 78a, 78b and 78c such as unusual temperature rise, drop and the like, had been observed, operational conditions of the fluidized bed polymerization reactor 70 was changed for stabilizing polymerization state.
However, when the temperature measuring devices 78a, 78b and 78c are located inside the fluidized polymerization vessel 70 by insertion for measuring the inner temperature thereof, the projected portion of a thermometer in the temperature measuring devices 78a, 78b and 78c will work as obstacle to convection of gas. Further, polymer particles will be adhered to and grown up on the temperature measuring devices 78a, 78b and 78c. Consequently, these polymer particles will be changed into sources of generating sheet or mass polymers. This will cause bad dispersion of the solid particles.
Namely, when many of the temperature measuring devices are arranged inside the fluidized bed polymerization vessel 70, this arrangement will invite residence of the solid particles around the temperature measuring device and lack of heat release, even though the solid particles in the fluidized bed region 70b are made to flow uniformly by dispersing uniformly fluid gas run into the gas introducing region 70a through the gas dispersing plate 79. Consequently, the solid particles will be adhered in a mass state to the inner wall of the fluidized bed polymerization vessel and the ratio of olefin particles welding together will increase.
Accordingly, such arrangement of the temperature measuring devices as had mentioned above, although it is indispensable to operation of the fluidized bed polymerization vessel, is inclined to cause non-uniform dispersion of the flowing solid particles, and also to cause instability of the fluidized bed. On this account, it is required to minimize the number of temperature measuring devices installed inside the polymerization vessel, especially, inside the fluidized bed.
However, the solid particles composed of catalytic particles and olefin polymer are floated in the fluidized bed region 70b so that the floated solid particles has their own distribution of temperatures, and positions at which heat spots appear are not fixed as well as their distribution is always changed. For that reason, it is very difficult to detect accurately the inner temperatures of the fluidized bed polymerization vessel 70. It is also very difficult to control exactly operational conditions of the fluidized bed polymerization vessel 70 based on this measurement.
As had mentioned above, the conventional temperature measuring method has disadvantages in that it is hard to detect speedily and exactly times and positions of heat spots occurring. Accordingly, it has been hitherto difficult to operate the fluidized bed polymerization vessel stably for a long period of time in succession.